I read the book freakonomics last week and remember when he was doing a story about prostitution that there were four factors in how well a job payed.
Supply of workers
How undesirable the job is
The demand for the job
And I forget the last one, does anyone know what it was? His example of this type of thing was prostitution, which pays very well. There is a very small supply of people willing to be prositutes, it is an undesirable job and there is alot of demand, as a result income is very high.